Depends how much oil is left in the engine. Dip it, and if it's light top it up. If the cap is missing, it's down to Halfords to bring one out to you, top up the engine, then take it away and clean it up, leaving you with transport in the meantime. Easy mistake for them to make, but there's no excuse. If they mess you about, tell them you will have an independent garage do it all, then sue them for the bill.